Epibiont growth on filamentous bacteria found in activated sludge: a morphological approach

Occurrence of epibiont attachment on filamentous bacteria is a common phenomenon in activated sludge. In this study, an attempt has been made to elucidate the intrinsic nature of the attachment between the epibionts and filamentous bacteria based on microscopic observations. Characterization of the epiflora based on fluorescence in situ hybridization using group level probes revealed that the epibionts colonizing these filamentous bacteria largely belongs to the class Alphaproteobacteria, followed by Beta and Gammaproteobacteria. The ultrastructural examination using transmission electron microscopy pointed to the existence of a possible cell-to-cell interaction between epibionts and the selected filaments. Common bacterial appendages such as pili and fimbria were absent at the interface and further noted was the presence of cell membrane extensions on epibiont bacteria protruding towards the targeted filamentous cell. Fibrillar structures resembling amyloid-like proteins were observed within the filament cells targeted by the epibionts. An interaction was apparent between amyloid such as proteins and epibionts with regards to the direction of fibrillar structures and the distance of approaching epibiont bacteria. Due to the lack of visual evidence in support of penetration, the role of these amyloid-like fibrils as potential attachment sites for the epibionts was taken into consideration, and required further validation using conformational antibodies.